linux-kernel - Re: [PATCH v4 1/2] tty: serial: dz: convert atomic_* to refcount_* APIs for map

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Date:   Tue, 10 Jan 2023 11:49:24 +0530
From:   Deepak R Varma <drv@...lo.com>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     "Maciej W. Rozycki" <macro@...am.me.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.com>,
        elena.reshetova@...el.com, ishkamiel@...il.com,
        keescook@...omium.org, dwindsor@...il.com,
        Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH v4 1/2] tty: serial: dz: convert atomic_* to refcount_*
 APIs for map_guard

On Tue, Jan 03, 2023 at 09:59:52AM +0100, Jiri Slaby wrote:
> On 26. 12. 22, 7:21, Deepak R Varma wrote:
> > The refcount_* APIs are designed to address known issues with the
> > atomic_t APIs for reference counting. They provide following distinct
> > advantages
> >     - protect the reference counters from overflow/underflow
> >     - avoid use-after-free errors
> >     - provide improved memory ordering guarantee schemes
> >     - neater and safer.
> 
> Really? (see below)
> 
> > --- a/drivers/tty/serial/dz.c
> > +++ b/drivers/tty/serial/dz.c
> ...
> > @@ -687,23 +686,19 @@ static int dz_map_port(struct uart_port *uport)
> >   static int dz_request_port(struct uart_port *uport)
> >   {
> >   	struct dz_mux *mux = to_dport(uport)->mux;
> > -	int map_guard;
> >   	int ret;
> > 
> > -	map_guard = atomic_add_return(1, &mux->map_guard);
> > -	if (map_guard == 1) {
> > -		if (!request_mem_region(uport->mapbase, dec_kn_slot_size,
> > -					"dz")) {
> > -			atomic_add(-1, &mux->map_guard);
> > -			printk(KERN_ERR
> > -			       "dz: Unable to reserve MMIO resource\n");
> > +	refcount_inc(&mux->map_guard);
> > +	if (refcount_read(&mux->map_guard) == 1) {
> 
> This is now racy, right?

Hello Jiri,
I found this [1] commit which introduced similar transformation in a
neighbouring driver. Can you please comment how is this different from the
current patch proposal?

[1] commit ID: 22a33651a56f ("convert sbd_duart.map_guard from atomic_t to refcount_t")

On a side note, I have not been able to find an exact 1:1 map to the
atomic_add_result API. I am wondering should we have one?

Thank you,
./drv


Thank you,
./drv

> 
> thanks,
> -- 
> js
> suse labs
>